Scientists Hike Probability of Major Quake

A panel of California’s leading earthquake scientists Monday revised upward the probability that a quake of magnitude 7 or greater will soon hit Southern California, saying there is a 5% to 12% chance of the quake each year and a 47% chance of it occurring within five years.

In the past, scientists said there was a 4% chance of such an earthquake in any given year. The likelihood of a damaging quake in the region has increased because of last June’s 7.5 magnitude Landers temblor and other recent seismic activity, they concluded.

The report by the scientists, convened by the U.S. Geological Survey and the state Office of Emergency Services, noted that still “the most likely case is that no large earthquake will occur” in the next few years. Nonetheless, the report was accompanied by calls for better quake preparedness.

If the quake does occur, it probably will be centered on the San Andreas Fault close to San Bernardino, Riverside or Palm Springs and cause more damage and casualties than the Landers earthquake, the report released Monday said.

The Landers temblor was centered in a thinly populated desert area, and the intensity of shaking in Southern California’s urban centers was not high enough to cause much damage.

But a quake of magnitude 7 or higher along southern segments of the San Andreas or on the San Jacinto Fault, another possibility cited in the report, would cause heavy shaking and structural collapses in the Inland Empire, according to various scenarios prepared by the scientific panel. Damage would be less heavy in Los Angeles, Orange and San Diego counties. The Ad Hoc Working Group on the Probabilities of Future Large Earthquakes in Southern California, chaired by Thomas H. Heaton of the U.S. Geological Survey and Keiiti Aki of USC, also said there is a 23% probability that a magnitude 6 or greater aftershock of the Landers and Big Bear quakes will occur by Sept. 1. The 6.6-magnitude Big Bear quake struck several hours after the larger Landers quake.

One of the group’s assessments--an 85% chance of an aftershock at magnitude 5 or higher--proved accurate Nov. 27 when a 5.4-magnitude temblor rattled the Big Bear area.

The scientists, however, cautioned against giving too much weight to earthquake probability estimates, which are based on historic quake patterns.

“Because few historical precedents exist and those that do are for rather different circumstances, probability cannot easily be addressed as frequency of occurrence or as a description of well-categorized randomness,” the report said. “Rather, probability must be interpreted as betting odds.”

Richard Andrews, director of the state Office of Emergency Services, Monday urged Southern California residents not to be “distracted” by the uncertainty of quake predictions. He said his agency takes the prospects seriously enough to update its mobilization plan for a major earthquake and is contemplating issuance of an “imminent alert” if such a quake appears to be in the offing.

A series of moderate earthquakes on the San Andreas Fault, for instance, might trigger such an alert. Scientists say that clusters of smaller quakes on the San Andreas can foretell a larger quake.

Los Angeles County Sheriff Sherman Block disclosed Monday that state disaster officials and scientists met last week with Los Angeles authorities to give a preview of the report and urge a step-up in local preparedness efforts.

At a news conference, Block, county Administrative Officer Richard B. Dixon and county Fire Chief Michael Freeman said official preparedness is good, but they urged individuals to prepare to live on their own for 72 hours after a major earthquake by stockpiling such supplies as food, water, bandages, flashlights and a battery-operated radio.

Block called individual preparedness in Southern California “fair at best,” and suggested that an earthquake emergency kit would make a good Christmas present for “someone you really care about.”

In the 42-page report--called “Future Seismic Hazards in Southern California”--the scientific panel pointed to four factors that “raise concern that a large earthquake might soon occur in Southern California.”

First, it said, the time between large quakes on parts of the San Andreas is already longer than the average known historic interval. “Portions of the southern San Andreas Fault appear ready for failure,” the report said.

Since 1985, the report said, the frequency of quakes in Southern California has run higher than in the previous four decades. The Landers quake is thought to have increased stress on parts of the San Andreas, and aftershocks of the Landers quake have recurred in seismic activity has historically been relatively low.

The scientists dismissed published speculation that the Landers quake signified a shift in tectonic stress to the Mojave Desert from the San Andreas Fault.

By far the greatest seismic threat to Southern California remains the San Andreas and the offshoot San Jacinto Fault, they said.

“Such earthquakes on the San Andreas and San Jacinto faults would cause much stronger shaking in more urbanized areas,” the report said. “The conclusions of this report underscore the plausibility of large damaging earthquakes affecting metropolitan areas of Southern California.”

One locality the scientists singled out for attention was Yucaipa, southeast of San Bernardino. “The (Landers) aftershocks near Yucaipa are a concern . . . and the strain field accompanying them merit careful monitoring,” they said.

The report also made these points:

* After terming the magnitude 6.6 Big Bear quake of June 28 a “twin” of the Landers quake, the scientists said they now have concluded that it was a Landers aftershock.

* “It is interesting to note,” the report said, “that in the 50 years preceding the great 1906 San Francisco earthquake, the rate of occurrence of moderate-sized earthquakes in Northern California was significantly greater than for the following several decades. Other studies of seismicity patterns further suggest that earthquakes tend to cluster in space and time.”

For instance, it noted, in 1812 two magnitude 7 earthquakes hit Southern California in just 13 days, one at Wrightwood and one offshore in the Santa Barbara Channel.

* In passing, the scientists noted that underground nuclear explosions can induce or trigger earthquakes, although they termed nuclear tests that took place June 19 and 23 in Nevada too small to be responsible for the Landers quake on June 28.

Monday’s report did not seek to re-evaluate a 30-year prediction made by a smaller scientific panel in 1988 that there was a 60% chance of a major quake occurring in Southern California by the year 2018. A later report will take another look at that prediction, the scientists said.

The report listed eight magnitude 7 or greater quakes occurring in Southern California in the 19th and 20th centuries. After the two magnitude 7 quakes in 1812, there was the 8.2 Fort Tejon quake in 1857, the 7.6 in the Owens Valley in 1872, the 7.3 near Lompoc in 1927, the 7.1 in the Imperial Valley in 1940, the 7.7 near Tehachapi in Kern County in 1952 and, finally, Landers.

The Most Likely Scenario

Leading seismologists said Monday that recent activity points to a higher risk of a large earthquake in Southern California than previously thought. The chance of a quake of magnitude 7 or greater in any year was put at 5% to 12%, compared to 4% under earlier scientific reports. The map shows the expected shaking if a quake of magnitude 7.8 were to rip along the San Andreas Fault from San Bernardino through the Coachella Valley, the scenario scientists called among the most likely.

Level of expected shaking in different areas, stated in terms used by the modified Mercalli scale, a method of describing the effects of a quake:

Level V: Felt by nearly everyone, many awakened. Cracked plaster, some windows broken.

Level VI: Felt by all; some run outdoors. Some damaged chimneys and fallen plaster, heavy furniture moved, overall damage slight.

Level VII: Frightens everyone. Considerable damage in poorly built structures, moderate in better buildings, negligible in well-constructed buildings. Some chimneys down.

Level VIII: Partial collapse and heavy damage of ordinary buildings. Walls, chimneys and factory stacks fall. Heavy furniture overturned. Driving disrupted by shaking.

Level IX: Buildings thrown off foundations, underground pipes broken, ground conspicuously cracked. Damage to buildings great, considerable even in specially designed structures.